A transponder for controlling an engine immobilizer of a motor vehicle is known from DE 195 33 309 A1. This conventional transponder relays a fixed code and alternating code to a transceiver unit in the vehicle, whereupon the engine immobilizer enables motor vehicle startup if both codes match expectations. The same transponder can also be used for opening doors equipped with a corresponding transceiver unit in the household area, wherein transmitting only the fixed code is to suffice for opening these doors. While consideration has been given to the capability of changing the fixed code transmitted by the key given a change in vehicles, this can only be accomplished with a significant outlay, and relies upon compliance with safety measures aimed at preventing an inadvertent change, resulting in a situation where the transponder can no longer be used for opening the doors in the household area.
Transponder systems for actuating door locks are in widespread use in the hospitality industry. The transponders in such systems are traditionally designed as plastic cards the size of check cards with a data storage. Before such a transponder is given to a hotel guest as the key to his or her room, a recorder loads it with access authorization information specific to the door of the room along with time limit information. When the guest holds the transponder up to a reader on the lock to his or her room door, both pieces of information are transmitted, and the lock is opened if the access authorization information matches the room door, and the transmission takes place within a period of validity specified in the time limit information, i.e., within the envisaged timeframe in which the guest uses the room. The lock will not open after this time has expired. If a guest stays longer than specified by the loaded time limit information upon being given the transponder, he or she must switch the transponder or have new information recorded at reception. If a departing guest fails to return the transponder, this known transponder system allows a new tenant to use the same system with another transponder, and opening the door without authorization using the unreturned transponder is impossible. The problem with rental cars or vehicles in a company fleet or car sharing pool is that each person who would like to use a vehicle traditionally needs a suitable key. Since each person with a key can use the vehicle to which the key belongs, it makes sense to the operator of a car fleet for there to only be a small number of keys to each vehicle. This allows him or her to keep the rather high costs for the keys within limits on the one hand and on the other hand eliminates the risk of conflicts between potential users of a vehicle if only a single user is in possession of a key. At the same time, too low a number of keys impedes the efficient use of the fleet, since one user can prevent another from using a vehicle by not returning his or her key when done. The known transponder system described above is not suitable for vehicles, since a user can only procure a usable transponder where the recorder is located. However, he or she cannot automatically just proceed to the location of the vehicle if a previous user has not parked it at the site of the recorder. However, it would be highly appealing in particular to users in a car sharing pool to be able to decide whether or not they wish to use a parked vehicle in the pool at the location where they find it, potentially doing so right away.